A monolithic optical injection-locked (MOIL) DFB laser with large stable injection locking range is experimentally demonstrated using the side-mode injection locking technique. The low-frequency roll-off in the MOIL DFB laser is suppressed significantly. The relaxation oscillation frequency is measured to be 26.84 GHz and the intrinsic 3-dB response bandwidth is more than 30 GHz, which is about 20 GHz higher than that of the free running DFB laser. The nonlinear distortions, including the 1-dB compression point, second harmonic distortion (2HD) and third-order intermodulation distortion (IMD3), are also suppressed significantly. A simple radio-over-fiber system transmitting 40 Msymbol/s 32-QAM signal with 6 GHz carrier is achieved using the MOIL DFB laser. After 50 km transmission, the average error vector magnitude (EVM) of the whole link is 2.94% in injection locked state, while the EVM in free running DFB laser is 5.25% as a comparison. To our knowledge, this is the first time that the MOIL DFB laser is realized utilizing the side-mode injection locking method.